tenant wants to assign/sub lease the tenancy to a tenancy at will/expired lease.

is this possible? im confused.
my dad has two tenants. one wants to assign the remainder of the lease agreement (3 out of the 7 years) to the other tenant (whose lease has expired, only need 1 m notice to vacate)...but that would be considered as a sub lease no???
and is transferring the whole or less of the remainder lease term to a "tenancy at will" possible? my understanding is no, but can somebody please confirm.
Thank you.